3D printing may be used to create three-dimensional objects by printing layers of material on top of previously laid-down layers of material, such that the finished or partially-finished 3D printed product is formed of a number of thin layers of material. 3D printing is sometimes alternatively known as “additive manufacturing” or “rapid prototyping”, all of which may refer to the general technique of producing 3D articles from printing multiple thin layers of the article, one on top of the other.
One 3D printing technique is to provide a movable stage beneath a computer-controlled print head. A thin flat layer of powder is spread over the stage and the printer head sprays a binder material onto specific points on the layer of powder, in accordance with a cross-section of the finished 3D printed article. The powder may be one of a variety of materials, for example a powdered metal or a powdered ceramic. The binder may be one of a variety of sprayable products, such as an adhesive. The binder reacts with the powder to form a solid blob of material at the point where the binder is sprayed. After the printer head has sprayed all the designated areas of the first layer of powder, the stage may be lowered and a new layer of loose powder is overlaid on the previous layer (that, after printing, comprises areas of loose powder and areas of bound powder). The printer head scans over the new layer of powder and deposits binder in accordance with the instructions from the controller. Thus, a second layer of the final article is completed. This process is repeated until all of the layers of the article have been printed.
Instead of using a binder from the printer head, a laser or electron beam may be used to bind the powder together. In this form of 3D printing, the laser (or electron beam) melts or welds together the metal particles caught within the beam. The beam is then scanned over the powder layer to form that cross-sectional layer of the finished or partially-finished article.
If a 3D printed article is printed using the above-described method, and the finished article contains internal passages, those passages may be filled with unbound powder. This powder must be removed to clear the passages and leave the desired article with internal passages properly formed.
After removal of the powder, the product may be complete or it may undergo further processing steps, such as heat treating.
If the powder was a metal powder, it may be desired to sinter the metal particles together to increase the strength of the article. In this process, the green (i.e. partially-completed) 3D printed article may be placed in a furnace and heated. Before heating, any excess loose powder should be removed so that the finished article properly matches the desired shape that was printed by the 3D printer. This is because a heat-treating step may fuse any unbonded powder that remains in the internal passages which may undesirably distort the internal passages. If the 3D printing process utilised a binder, then the binder may have been selected to thermally decompose during heating to allow the metal powder particles to sinter together firmly.
One type of article that may be made by 3D printing is a heat exchanger. Heat exchangers typically have a variety of internal passages for a first fluid (e.g. refrigerant) which passages may themselves surround or partially surround external passages for a second fluid (e.g. air or a second refrigerant). When a heat exchanger is constructed through 3D printing, there may be many passages, both internal and external, containing loose powder. The loose powder requires a clear access path to be properly removed, which complete heat exchanger assemblies often do not have due to the complex nature of the flow paths that have been designed for the ideal thermal performance and flow distribution. If this loose powder is not removed, it may solidify during any subsequent heat treatment and hence block the heat exchanger's flow channels thereby affecting the performance of the unit. Further, as the internal passages of the heat exchanger cannot be easily inspected it may be difficult to determine with confidence whether any cleaning process has been successful in completely removing loose powder.
Thus, there is a need for methods and systems for removing loose powder from 3D printed articles, including removing powder from internal paths within the green article or the finished article. Various methods for cleaning loose powder from internal passages have been proposed in the prior art.
US 2016/0074940 discloses a method of removing loose powder material from a cavity within a 3D printed part, wherein the cavity has at least one opening leading to the outside of the part. The part is placed on a stage and vibrated until the powder fluidizes and flows out of the cavity via the opening(s).
Such conventional methods have generally been considered satisfactory for their intended purpose. However, there is a need in the art for improved methods of clearing powder from 3D printed articles, particularly heat exchangers.